1. Field of the Invention
The present invention relates to an apparatus and a process for a chemical vapor deposition of a compound film on a substrate, particularly to an apparatus and a process which enable a stable growth of a compound film on a substrate with an improved reproductivity.
2. Description of the Related Art
In the chemical vapor deposition (CVD) of semiconductor films from a solid source having a low vapor pressure, or in the CVD of superconductor films, most of such films are formed by a process in which a solid source is heated to raise the vapor pressure and the thus-generated gaseous source is used for the deposition of a film. Most CVD apparatuses are assembled as an integral unit including a gas-generation part and a film-growth part, because of the required transportation of a hot gas.
FIG. 1(a) shows a vertical section of a conventional horizontal CVD apparatus comprising a reaction tube 10, a source chamber 11, source boats 12a, 12b, 12c, and 12d, in which respective solid sources are heated and evaporated, electric resistance heaters 13a, 13b, 13c, 13d, and 13e, carrier gas inlets 14a, 14b, 14c, and 14d through which a carrier gas such as He is introduced, a gas inlet 14e through which a necessary gaseous source such as O.sub.2 is introduced, a substrate 15, a substrate holder 16 holding the substrate 15, and flow-paths 17a, 17b, 17c, and 17d for gases generated by an evaporation of solid sources contained in the source boats 12a to 12d; the paths 17a to 17d also functioning as entrances for the introduction, replacement, and replenishment of the solid sources in the source boats 12a to 12d.
The electric resistance heaters 13a to 13d are provided for heating solid sources contained in the source boats 12a-d, respectively and the heater 13e is provided for heating the substrate 15. The source chamber 11 in the reaction tube 10 has a multiple-floor construction forming the divided gas paths 17a to 17d, and the source boats 12a to 12d respectively containing solid sources, are located at the predetermined positions respectively within the gas paths 17a to 17d.
In the conventional CVD apparatus as shown in FIG. 1(a), the source chamber 11 is situated inside the reaction tube 10 and the source boats 12a to 12d containing solid sources are situated in the gas paths 17a to 17d, and compound film is formed on the substrate 15 by using the gases generated by heating and evaporating the respective solid sources. Most conventional CVD apparatus of, for example, a Bi-Sr-Ca-Cu-O oxide superconductor, are a horizontal type such as shown in FIG. 1, in which gaseous sources are generated inside the apparatus and a Bi-Sr-Ca-Cu-O oxide superconductor film is formed on the substrate 15.
In such conventional CVD apparatuses, solid sources are placed at respective predetermined positions in the gas paths 17a to 17d formed as multiple floors inside the source chamber 11, and the gases generated at the respective solid source positions are entrained in and transported by a carrier gas to reach the substrate 15 and form a compound film thereon.
This conventional arrangement, however, does not ensure a stable and reproductive formation of a compound film on the substrate 15, due to an unsatisfactory control of the heating temperatures for the solid sources and to an insufficient mixing of the gases generated by the evaporation of the solid sources. This is because, first the heating temperature of a solid source is significantly affected by the position at which the corresponding source boat is situated, since it is practically very difficult to allow a long uniform heating temperature zone, as this would require a practically unacceptable long length of the reaction tube 10 which is usually made of a gastight refractory material such as quartz. FIG. 1(b), in which portions P1 and P2 correspond to positions P1 and P2 of FIG. 1(a), depicts a typical heating temperature distribution through the regions corresponding to the heaters 13a to 13e, in which the significantly different required heating temperatures of 160.degree., 450.degree., and 840.degree. C. for the source boats 12a to 12c are only properly established within very short zones, respectively, although the required heating temperature of 850.degree. C. for the boat 12d and the substrate 15 is established over a relatively long zone. Such a non-uniform or graduated distribution of the heating temperature within each of the respective heater regions occurs because each heating temperature is influenced by the neighboring heating temperature, and thus abrupt changes of heating temperatures cannot be practically realized. Accordingly, actual temperatures of the corresponding solid sources depend greatly upon the positions at which the solid source boats 12a to 12d are located, which means that the control of the evaporation from the solid sources is very difficult, and thus it is not possible to obtain a stable reproductive formation of a compound film on the substrate 15.
It is also difficult to individually adjust the concentrations of generated source gases, one to the other, since when a preset temperature of a heater is changed to change the concentration of the generated or evaporated source gas, the neighboring heating temperatures also are unavoidably varied, and thus the concentration of the corresponding source gases are varied.
Second, in the conventional arrangement a laminar flow of gas above the substrate 15 tends to occur due to the multiple floor form of the gas paths 17a to 17d, and accordingly, measures have been taken to obtain a uniform mix of the source gases, e.g, a mixing means is provided at the exits of the gas paths 17a to 17d to ensure a proper mix of the source gases and adjust any fluctuations in the flow of the source gases. Upon replenishment or replacement of the solid sources in the source chamber 11, however, the mixing means associated with the gas paths 17a to 17d must be removed, which is time and labor-consuming. A gas-mixing means is provided in particular to obtain a uniform gas concentration around the substrate 15. Usually, in addition to the provision of a mixing means, if the exit portions of the gas paths 17a-d are arranged in a complicated manner, a more uniform the concentration of gas around the substrate 15 can be obtained, and in turn, a more stable compound film formed thereon. This, however, complicates the operations of removing the mixing means, etc., and thus the replacement and replenishment of the solid sources become more difficult. Although it may be possible to replace and replenish the solid sources at an upstream portion of a conventional CVD apparatus, such operations are practically very difficult, since the carrier gas inlets 14a-d are usually too narrow to allow the insertion of solid sources.